1. Field of the Invention
The present invention relates to a liquid crystal display device and, more specifically, to a liquid crystal display device in which a pair of insulating substrates are opposed to each other via a predetermined gap that is maintained by spacers, a liquid crystal composition (a liquid crystal molecule) is held in the gap, and a storage capacitor portion is formed in each pixel region.
2. Description of the Related Art
High-resolution liquid crystal display devices capable of color display for use in notebook-sized computers and computer monitors are now widely utilized.
Basically, in this type of liquid crystal display device, what is called a liquid crystal panel is formed by holding a layer of a liquid crystal composition between two insulating substrates (hereinafter also referred to simply as substrates) such as glass plates at least one of which is transparent. This type of liquid crystal display device is generally classified into a type (simple matrix type) in which an image is formed by changing the orientation directions of liquid crystal molecules of desired pixels by selectively applying voltages to various electrodes for pixel formation that are formed on the insulating substrates of the liquid crystal panel and a type (active matrix type) in which various electrodes for pixel formation and active elements for pixel selection are formed and an image is formed by changing the orientation directions of liquid crystal molecules of desired pixels by making selections from the active elements.
In general, the active matrix liquid crystal display device employs that is called a vertical electric field type in which electric fields are developed between electrodes formed on one substrate and an electrode formed on the other substrate.
On the other hand, the liquid crystal display device of what is called a lateral electric field type (also called as In-Plane Switching type, abbreviated as xe2x80x9cIPS typexe2x80x9d hereinafter) has been put into practical use in which the directions of electric fields that act on the liquid crystal layer are approximately parallel with the substrate surfaces. In an example of the lateral electric field type liquid crystal display device, a very wide viewing angle is obtained by forming comb-teeth electrodes for electric field formation on one of the two substrates.
In the lateral electric field liquid crystal display device, an active matrix substrate is provided with scanning signal lines and video signal lines, switching elements formed in the vicinity of the crossing points of the scanning signal lines and the video signal lines, pixel electrodes to which drive voltages are applied via the respective switching elements, and counter electrodes that are formed in the same plane as the pixel electrodes. A color filter substrate is provided with a black matrix made of a resin composition and color filter layers formed for each pixel in an opening region of the black matrix. A liquid crystal panel is formed by holding a liquid crystal composition between the active matrix substrate and the color filter substrate. The liquid crystal display device is configured in such a manner that a backlight is disposed in the rear of the liquid crystal panel and a unified structure is obtained by using top and bottom cases.
Image display is performed by changing the light transmittance of the liquid crystal compound by electric field components that are formed between the pixel electrodes and the counter electrodes so as to extend approximately parallel with the substrate surfaces.
In contrast to the vertical electric field type one, the lateral electric field type liquid crystal display device is superior in viewing angle; that is, it allows a user to view a clear image even when he is located at a position that forms a large angle with the display screen.
The liquid crystal display device having the above configuration is disclosed in Japanese Unexamined Patent Publication No. Hei. 6-160878 and its counterpart U.S. Pat. Nos. 5,598,285, and 5,737,051, for example.
FIG. 1 is a plan view showing one pixel, a light shield region of a black matrix BM, and its vicinity of a conventional lateral electric field type liquid crystal display device.
As shown in FIG. 1, each pixel is provided in a region enclosed by four signal lines that cross each other, that is, a scanning signal line (gate signal line or horizontal signal line) GL, a counter voltage signal line CL, and two adjacent video signal lines (drain signal line or vertical signal line DL.
Each pixel includes a thin-film transistor TFT, a storage capacitor portion Cstg, a pixel electrode PX, and an counter electrode CT. In FIG. 1, a plurality of scanning signal lines GL and counter voltage signal lines CL are provided at the top and bottom of the pixel direction so as to extend in the right-left or horizontal direction. A plurality of video signal lines DL are provided at the right-left side of the pixel so as to extend in the top-bottom or vertical direction. The pixel electrode PX is connected to the thin-film transistor TFT, and the counter electrode CT is integral with the counter voltage signal line CL.
The pixel electrode PX and the counter electrode CT confront each other, and display is controlled by modulating transmission light or reflection light by controlling the orientation state of a layer of a liquid crystal composition LC (hereinafter also referred to simply as a liquid crystal or a liquid crystal layer) by means of an electric field developed between the pixel electrode PX and the counter electrode CT. Each of the pixel electrode PX and the counter electrode CT assumes a comb-teeth shape and has long and narrow portions extending in the top-bottom or vertical direction in FIG. 1.
The pixel electrode PX and the counter electrode CT are formed in such a manner that the number P of comb-teeth portions of the pixel electrode PX and number C of comb-teeth portions of the counter electrode CT in one pixel necessarily satisfy a relationship C=P+1 (in FIG. 1, C=2 and P=1). The comb-teeth portions of the counter electrode CT and those of the pixel electrode PX are arranged alternately so as to have the comb-teeth portions of the counter electrode CT arranged adjacent to the video signal lines DL. With this structure, shielding from electric lines of force originating from the video signal lines DL can be effected by the counter electrode CT so that electric fields between the counter electrode CT and the pixel electrode PX are not influenced by the electric fields originating from the video signal lines.
The potential of the counter electrode CT is stable because it is always supplied with a potential externally via the counter voltage signal line CL. Therefore, the counter electrode CT has almost no potential variation even if it is adjacent to the video signal lines DL. Further, the above structure makes the geometrical position of the pixel electrode PX more distant from the video signal lines DL, whereby the parasitic capacitances between the pixel electrode PX and the video signal lines DL are greatly reduced and hence a variation of a pixel electrode potential Vs due to video signal voltages can be controlled. As a result, vertically extending crosstalk lines (an image quality failure called vertical smears) can be prevented.
In a specific construction, widths Wp and Wc of the pixel electrode PX and the counter electrode CT, respectively, are set at 6 xcexcm, which is sufficiently larger than 4.5 xcexcm which is the maximum setting thickness of a liquid crystal layer (described later). It is desirable that the electrode widths Wp and Wc be sufficiently larger than 5.4 xcexcm because it is preferable to provide a margin of 20% or more in view of processing variations in manufacture. As a result, electric field components parallel with the substrate surfaces that are applied to the liquid crystal layer become stronger than those perpendicular to the substrate surfaces, which prevents voltages for driving the liquid crystal to become unduly high. It is preferable that the maximum values of the electrode widths Wp and Wc be smaller than an interval L between the pixel electrode PX and the counter electrode CT. This is because it the interval between the electrodes is too short, electric lines of force are curved sharply and hence regions where electric field components parallel with the substrate surfaces are stronger than those perpendicular to the substrate surfaces are made larger, as a result of which the electric field components parallel with the substrate surfaces cannot be applied to the liquid crystal layer efficiently. To give a margin of 20% to the interval L between the pixel electrode PX and the counter electrode CT, it is necessary that the interval L be larger than 7.2 xcexcm. For example, in a case of constructing a liquid crystal display device having a diagonal size of about 14.5 cm (5.7 inches) and a resolution of 640xc3x97480 dots, an interval L that is larger than 7.2 xcexcm can be realized by setting the pixel pitch at about 60 xcexcm and dividing each pixel into two parts.
To avoid disconnection, the electrode width of the video signal lines DL is set at 8 xcexcm, which is somewhat larger than the widths of the pixel electrode PX and the counter electrode CT. To avoid short-circuiting, an interval of about 1 xcexcm is provided between the video signal lines DL and the counter electrode CT. The video signal lines DL and the counter electrode CT are provided in different layers by forming the video signal lines DL and the counter electrode CT above and below a gate insulating film, respectively. On the other hand, the interval between the pixel electrode PX and the counter electrode CT is changed in accordance with the liquid crystal material used, for the following reason. The electric field intensity for attaining the maximum transmittance depends on the liquid crystal material. To obtain the maximum transmittance within the range of the maximum amplitude of a signal voltage that is set by the breakdown voltage of a video signal driver circuit (signal-side driver) used, the electrode interval needs to be set in accordance with the liquid crystal material. The electrode interval becomes about 15 xcexcm when a liquid crystal material that will be described later is used.
In the example configuration being discussed, in a plan view of FIG. 1, a black matrix BM surrounds an opening of the pixel and is formed on the gate line GL, the counter voltage signal line CL, the thin-film transistor TFT, and the drain lines DL, and between the counter electrode CT and the drain lines DL. The storage capacitor portion Cstg is located outside the opening of the black matrix BM (i.e., outside the pixel region) and is composed of the pixel electrode PX, the counter voltage signal line CL, and an insulating film formed between them.
In the liquid crystal display device, an alignment film is applied after formation of the respective electrodes and electrode wiring lines, protective films, and insulating films and is given a liquid crystal alignment control ability by being subjected to a treatment called rubbing.
In the conventional lateral electric field type liquid crystal display device, since the storage capacitor portion Cstg is formed outside each pixel region, there is no large hight change or steps in the pixel region and hence the alignment film in the pixel region can be given a uniform liquid crystal alignment control ability.
However, in recent years, liquid crystal display devices have been proposed in which the aperture ratio of the entire screen is increased by forming the storage capacitor portion Cstg in each pixel region. In those devices, the storage capacitor Cstg produces large steps in the pixel region and those steps may cause an alignment defect in a rubbing treatment. As a result, what is called a domain occurs and causes display unevenness.
In particular, alignment defects of the above kind occur frequently in a case where the multilayered film structure that constitutes the storage capacitor portion Cstg has steps extending perpendicularly or approximately perpendicularly to the alignment direction (rubbing direction) of the alignment film. When such an alignment defect occurs, the liquid crystal does not operate normally in the vicinity of the storage capacitor portion Cstg, to cause a domain. This results in a problem that the contrast is lowered and display unevenness occurs, which means a marked reduction in image quality.
An object of the present invention is to provide a liquid crystal display device having good display quality by solving the above problems in the art, specifically by decreasing the frequency of occurrence of alignment defects that result from the presence of steps of the storage capacitor portion Cstg formed in each pixel region and thereby preventing display failures such as a contrast reduction and display unevenness.
To attain the above object, in the invention, edge sectional shapes of steps of a multilayered film structure formed in each pixel region (actually each opening region of a black matrix), in particular steps of multilayered films such as electrode that constitute a storage capacitor portion, are made gentle.
Specifically, the invention provides a liquid crystal display device comprising an active matrix substrate comprising a plurality of scanning signal lines, a plurality of video signal lines, switching elements formed in the vicinity of respective crossing points of the scanning signal lines and the video signal lines, pixel electrodes to which drive voltages are applied via the respective switching elements, counter electrodes formed in a different plane than the pixel electrodes; a color filter substrate comprising a black matrix made of a resin composition, and color filter layers provided for respective pixels formed in respective opening regions of the black matrix; a liquid crystal composition held between the active matrix substrate and the color filter substrate; and storage capacitor portions located in the respective opening regions of the black matrix, each of the storage capacitor portions being composed of a counter voltage signal line for supplying a signal to an associated one of the counter electrodes, a pixel electrode, and an insulating film provided between the counter voltage signal line and the pixel electrode, wherein image display is performed by varying the light transmittance of the liquid crystal composition by electric field components that develop between the pixel electrodes and the counter electrodes so as to extend approximately parallel with the substrate surfaces, and wherein one of a portion of the counter voltage signal line and a portion of the pixel electrode which form the storage capacitor have an outline within that of the other of the portion of the counter voltage signal line and the portion of the pixel electrode.
With the above configuration, the angles of steps in the storage capacitor portions with respect to the surface of an alignment film become gentle, whereby rubbing defects can be prevented from occurring in the vicinity of the storage capacitor portions. As a result, it becomes possible to provide a liquid crystal display device having good display quality in which the frequency of occurrence of display failures such as a contrast reduction and display unevenness are greatly reduced.
In the above liquid crystal display device, most of a projected outline shape of the portion of one of the counter voltage signal line and the pixel electrode arranged in an upper layer may be located inside a projected outline shape of the portion of the other of the counter voltage signal line and the pixel electrode arranged in a lower layer.
In the above liquid crystal display device, most of a projected outline shape of the portion of one of the counter voltage signal line and the pixel electrode arranged in a lower layer may be located inside a projected outline shape of the portion of the other of the counter voltage signal line and the pixel electrode arranged in an upper layer.
With the above two features, the angles of steps of the electrode films constituting the storage capacitor portions with respect to the surface of an alignment film become gentle, whereby rubbing defects can be prevented from occurring in the vicinity of the storage capacitor portions. As a result, it becomes possible to provide a liquid crystal display device having good display quality in which the frequency of occurrence of display failures such as a contrast reduction and display unevenness are greatly reduced.
These features can be not only applied to the pixel electrode and the pixel, but also to a pair of conductive layers extending in transverse directions and crossing one another in the pixel region and at least one of the conductive layers including the branch portion extending from the crossing in the direction of extension of the other of the conductive layers within the pixel region. The pixel region is defined as a region formed on the liquid crystal display substrate and transmitting light to be modulated by the liquid crystal layer.
Where multilayered films such as electrode films, insulating films, and protective films are formed in each pixel region, the invention is not limited to the case where the multilayered films are ones belonging to the storage capacitor portion.
The second object of the invention is to prevent the liquid crystal display device from display failures caused by the reduction of the voltage applied to its liquid crystal layer. This problem appears the liquid crystal display having such two kinds of conductive layers defined as first and second conductive layers as follows. The first and second conductive layers are formed above a main surface of one of substrates facing to the liquid crystal layer. The first conductive layer extends in first direction, and has a first voltage. The second conductive layer extends in second direction, and has a second voltage. Each of the first and second conductive layers has at least one portion therefrom in a different direction and joined thereto at a corner, and is covered with an insulating film.
When the first voltage differs from the second voltage, the display failures appear in the liquid crystal layer. Namely, applying the different electrical signals or voltages the first and second conductors induces this problem, even if the first and second conductive layers are isolated from one another by an insulating film, or are formed on the main surfaces of different substrates from one another.
To attain this object, the invention forms the corner with at least one of a curve and at least one obtuse.
Specifically, the invention provides following two structures for the liquid crystal display device having a pixel electrode, an counter electrode, and an counter voltage signal line for supplying a signal to the counter electrode formed between a liquid crystal layer and one of a pair of substrates sealing the liquid crystal layer therebetween.
One of the structures is described that the counter voltage signal line extending in one direction crosses the pixel electrode extending in a transverse direction, at least one of the counter voltage signal line and the pixel electrode has branch portions extending in the direction of extension of the other the counter voltage signal line and the pixel electrode, and edges of the counter voltage signal line and the pixel electrode with the branch portions thereof are connected with at least one of a curve and at least one obtuse angle.
The other of the structures is described that the counter voltage signal line extending in one direction is connected to the counter electrode extending in transverse direction to the one direction so as to connect their edges with at least one of a curve and at least one obtuse, and the counter voltage signal line and the pixel electrode cross one another.
In both of the inventions, the pixel electrode, the counter electrode, the counter voltage signal line, and the substrate on which these electrodes and the signal line are covered with an insulating layer, and preferably the insulating film formed on the pixel electrode is different from that formed on the counter electrode and the counter voltage signal line. The crossing region in accordance to an overlapping of the counter voltage signal line and the pixel electrode is formed in the light transmission region delimited by a light shielding material.
The detailed structures of this invention will be explained later.